Just look at those guys mentioned that are his remaining top free agents. Almost all have some risk or concerns with them.
Arietta - love what he did with the Cubs and I'd like to have him back on a reasonable 3 or 4 year deal as I think it's best for both parties knowing the struggles he had with control before coming to the Cubs plus his velocity went down last season. He turns 32 this year so there is question how much longer can he perform at a high level to justify paying him like an ace.
Martinez - right in his prime at age 30 coming off a career year, I think the Red Sox will eventually pay him but Boras is probably pricing him too high for him to still be available. He probably carries the least amount of risk than any guy on that list
Hosmer - just 28 but 1B is a place a lot of teams are already set at and he's never hit more than 25 HR in a season yet which 1B is a position most associate as a power hitting positon. He tied for 18th amongst all 1B for HR last season, will be interesting to see where he winds up and for how long of a deal.
Moustakas - 29 years old coming off a career year of 38 HR 85 RBI. Not exactly a top end 3B though with just a 1.8 WAR which ranks him 22 amongst all MLB 3B last season.
Holland - 32 year old closer, he missed all of 2016 after having Tommy John surgery. He had a good start to 2017 but if you look at his pre and post all star break splits he had a horrible 2nd half of the season and it carried into the postseason as well so have to wonder about his durability plus his velocity was down last year too. Boras probably values him as a high end closer, he probably will have to settle for a 1 or 2 year deal at far less than what Boras values him at. Just did a google search on him and 2 of the first links that popped up were articles why the Nationals or Rockies should avoid signing him so the fact he's still available says a lot about how multiple teams are valuing him at right now
